U.S. Pat. No. 7,460,377 discloses (with reference to FIGS. 1 and 2) a converter 1 comprising fifteen input lines 2 connected to three output lines 3 and comprising five converting groups 4. Each of the five converting groups 4 comprises a first converting stage 5 with switches connected to the input lines 2, and a second converting stage 6 with switches connected to the output lines 3. For each converting group 4, all the switches of the first stage 5 are connected to all the switches of the second stage 6. The switches are bidirectional switches driven by a control system.
This converter has some drawbacks. In fact, during operation, the control system must avoid that input lines of the same converting group 4 are operated at different polarities; in other words it must be avoided that, for a converting group, an input line for example 2a is fed with positive input voltage and another input line for example 2b is fed with a negative voltage and the switches of both input lines 2a and 2b conduct, because this would cause a short circuit.
With reference to FIG. 2, for safety reasons when switching from a positive voltage to a negative voltage or vice versa a safety zone 8 at 0 voltage is provided.
For this reason, the voltage (and in particular its harmonics) at the output lines is not fully optimized according to the needs and the control system and the software that implements it can be complicated.